1. Field of the Invention
The present invention relates to image forming apparatuses such as copying machines, printers, facsimiles, etc. and a developing device for use in such image forming apparatuses. The present invention more particularly relates to a developing device and an image forming apparatus using the developing device that uses a two component developer including a toner and a magnetic carrier and that includes a self toner density control device to control the toner density of the developer on a developer bearing member so as to be within a fixed range without using a complex toner replenishing device.
2. Discussion of the Background
An example of a developing device that controls the density of toner in the developer on a developer bearing member without using a complex toner replenishing device is one in which the quantity of developer carried on and conveyed by a developer bearing member, that has an internally disposed magnetic field generating device, is regulated by a developer regulating member. The developer scraped off by the developer regulating member is put in a developer accommodating unit, and toner accommodated in a toner accommodating unit is supplied to the developer carried on the developer bearing member through a toner supplying opening, which is adjacent to the developer accommodating unit upstream in a developer conveying direction and which opposes a surface of the developer bearing member (see e.g. Japanese Laid-open Patent publication No. 9-22178). In this developing device, the toner contacting the developer carried on and conveyed by the developer bearing member at the toner supplying opening is taken into the conveyed developer at the interface between the conveyed developer and the developer in the developer accommodating unit. Once the toner density of the developer on the developer bearing member reaches a certain level, the volume of the developer accommodated in the developer accommodating unit starts to swell and thereby the developer in the developer accommodating unit extends so as to cover the toner supplying opening. As a result, the toner is stopped from being taken into the developer on the developer bearing member from the toner accommodating unit at the toner supplying opening. Thus, the toner density of the developer on the developer bearing member is controlled to be within a fixed range.
Also, a developing device is known in which a second developer regulating member is provided between the developer accommodating unit and the toner supplying opening of the toner accommodating unit so as to have a predetermined gap relative to a surface of a developer bearing member (see e.g. Japanese Laid-open Patent No. 9-197833). In this developing device, as the toner density of the developer on the developer bearing member increases, the thickness of a developer layer on the developer bearing member increases, and the increased portion of the developer layer is regulated by the second developer regulating member to pass the second developer regulating member. The developer thus regulated by the second developer regulating member covers the toner supplying opening located adjacent to and upstream of the second developer regulating member in the developer conveying direction, and thereby the toner is stopped from being taken into the developer being carried and conveyed by the developer bearing member. As a result, the toner density of the developer on the developer bearing member is controlled to be within a fixed range.
Such developing devices as noted above that control the toner density of the developer on a developer bearing member to be within a certain range (such a control is hereinafter referred to as a self toner density control) have an advantage in that a toner supplying unit can be simplified, because a toner density sensor is not required, and further a stirring device such as a paddle screw for stirring the developer can be eliminated. In addition, compared to a two component developing method, less magnetic carrier is required. Therefore, torque of the apparatus can be greatly reduced. From these points, the apparatus can be made compact and the cost of the apparatus reduced. Furthermore, because the toner in the toner accommodating unit can be directly supplied to the developer on the developer bearing member passing a developing area, the toner is promptly supplied to a part of the developer on the developer bearing member, where the toner density has been reduced by passing through the developing area. Accordingly, the developing devices can develop a solid image without decreasing the image density.
However, in such developing devices with a self toner density control capability, background soiling and decrease of image density occasionally occur with the lapse of time. Also, uneven image density sometimes occurs in a widthwise direction of an image, which direction is perpendicular to the direction in which the developer on the developer bearing member is conveyed (hereinafter referred to as the image width direction). The inventors of the present invention have analyzed the causes of the decrease in the image quality, and have found that the followings are some of the possible causes.
First, background soiling and decrease of image density with the lapse of time might be caused by an early deterioration of developer due to insufficient replacement of the developer on a developer bearing member with the developer in a developer accommodating unit. That is, if the developer carried on the developer bearing member is conveyed without being peeled off the developer bearing member at all, the developer on the developer bearing member is hardly replaced with the developer in the developer accommodating unit. If such replacement of the developer is not made, stress to the magnetic carrier in the developer carried on the developer bearing member is increased, which may lead to a deterioration of the magnetic carrier earlier than the magnetic carrier in the developer accommodated in the developer accommodating unit. If the magnetic carrier on the developer bearing member deteriorates earlier than the magnetic carrier in the developer in the developer accommodating unit, even when the developer as a whole in the developing device is not deteriorated so much, the charging capability of the magnetic carrier on the developer bearing member, that acts on a formation of a toner image, might be decreased. The decrease of the charging capability of the developer on the developer bearing member causes background soiling or decrease in the image density, thereby decreasing the image quality. Accordingly, the developer is forced to be replaced earlier than desired.
If a replacement of the developer on a developer bearing member with the developer in a developer accommodating unit is not sufficiently performed, inferior toner charging may also result. Namely, when such a replacement of the developer is not made, the toner in an amount corresponding to a consumed amount of the toner on the developer bearing member, which has been supplied onto the developer bearing member, must be mixed thereupon with the developer so as to be charged. However, when the surface moving speed of the developer bearing member is relatively fast (e.g., several hundred rpm when the developer bearing member is a roller), the toner can not be sufficiently charged in time for development, thereby causing an inferior charging of the toner. In particular, when much toner has been consumed at some parts of the developer bearing member, decrease of image density remarkably appears in such parts compared to other parts, thereby leading to a decrease of the image quality.
Secondly, uneven image density in the width direction of the developer bearing member (i.e., in the image width direction) might be caused by uneven toner charging in the width direction of the developer bearing member. When new toner is supplied to a part of the developer where toner has been consumed, supplying of the new toner might be delayed or the supplying quantity of the new toner might be varied. Such a delay in supplying new toner or an uneven new toner supplying amount causes a decrease in the toner density of the developer at the corresponding parts of the developer, and as a result the toner charging amount increases. In particular, when an image consuming a large amount of toner, such as one having a solid part, is formed, supplying of a sufficient amount of toner to a part of the developer, where the large amount of toner has been consumed, can not be made in time for a next development, and the next development might be performed with developer with decreased toner density at that part of the developer. Also, because toner is rather rapidly taken into a part of the developer where a large amount of toner has been consumed, extra toner may be unnecessarily taken into the part of the developer, leading to a development of a subsequent image with unnecessarily high toner density at that part of the developer.
Further, because toner is not consumed at non-image parts of an image, toner should not be taken into such parts of the developer corresponding to the non-image parts of the image. However, a small amount of the toner might be also taken into such parts of the developer corresponding to the non-image parts of the image, thereby causing an increase of the toner density and resulting in a decrease of the toner charging.
Thus, if uneven toner density or uneven toner charging occurs in the developer on the developer bearing member in the width direction of the developer bearing member, uneven image density is caused in the width direction of the developer bearing member such that a part of the image corresponding to a part of the developer where the toner density is low and the toner charging is high is relatively faint, forming a white strip, and a non-image part corresponding to a part of the developer where the toner density is high and the toner charging is low is relatively dark.
In addition, uneven image density in the width direction of the developer bearing member might be also caused by a variation in the dimension or position of a toner supplying opening of a toner accommodating unit in the width direction of the developer bearing member or by a variation of a magnetic force of a magnetic roller, that is provided inside of the developer bearing member as a magnetic force generating device, in the width direction of the developer bearing member.
Furthermore, the inventors of the present invention have also found that uneven image density in the width direction of a developer bearing member might be also caused by surface roughness of a part of a second developer regulating member opposing the developer bearing member or by an insufficient pressing force of an agitator provided in a toner accommodating unit for pressing the toner to be conveyed. Specifically, the surface roughness of the part of the second developer regulating member opposing the developer bearing member affects the quantity of toner to be taken into the developer. If the surface roughness is uneven in the width direction of the developer bearing member, the quantity of toner to be taken into the developer becomes uneven in the width direction of the developer bearing member, and thereby uneven image density is caused in the width direction of the developer bearing member. The toner pressing force of the agitator in the toner accommodating unit also affects the quantity of toner to be taken into the developer. If the toner pressing force of the agitator is uneven in the width direction of the developer bearing member, and when the toner pressing force is not sufficient to convey toner to a part of the developer where a large amount of toner is necessary for forming, for example, a solid image, the toner cannot be sufficiently taken into such a part of the developer, thereby causing uneven image density in the width direction of the developer bearing member.
The inventor of the present invention has proposed a developing device having a second developer regulating member, in which a developer peeling off member for peeling off the developer on a developer bearing member and a developer stirring member for stirring the developer are provided in a developer accommodating unit for preventing the above-discussed uneven image density from occurring (see e.g. Japanese Laid-open Patent Publication No. 10-232540). In that developing device, all of the developer carried and conveyed by the developer bearing member inside the developer accommodating unit is once peeled off by the developer peeling off member and the peeled off developer is stirred by the stirring member. According to a result of an experiment recently made by the inventors of the present invention with the above developing device, it has been found that the self toner density control is not stable enough and that in the worst case the self toner density control does not appropriately function. Specifically, because all of the developer on the developer bearing member is peeled off in the developer accommodating unit, the peeled off developer is accommodated in the developer accommodating unit one after another until the space in the developer accommodating unit is filled with the peeled off developer. Therefore, when the toner density is increased, it is hard to suppress a movement of the developer on the developer bearing member at the upstream side of the developer peeling off member in the developer conveying direction, such that a developer accumulated portion is not formed as desired upstream of the second developer regulating member in the developer conveying direction.
The present invention has been made in view of the above-discussed and other problems and addresses the above-discussed and other problems.
Preferred embodiments of the present invention provide a novel developing device and image forming device using the developing device, which prevent background soiling and decrease of image density due to a lapse of time while maintaining a stable self toner density control, and which at the same time perform a satisfactory development without causing uneven image density in the width direction of a developer bearing member, which is perpendicular to a direction in which the developer on the developer bearing member is conveyed.
Preferred embodiments of the present invention also provide a novel developing device and image forming apparatus using the developing device, which performs a satisfactory development without causing uneven image density in the width direction of the developer bearing member while keeping a stable self toner density control.
According to a preferred embodiment of the present invention, a developing device includes a developer bearing member which includes an internal magnetic field generating device and which is configured such that a surface thereof moves so as to carry and convey a developer including toner and carrier. A developer regulating member is configured to regulate a quantity of the developer carried and conveyed by the developer bearing member toward a developing area, and a developer accommodating unit is configured to accommodate the developer regulated to be conveyed toward the developing area by the developer regulating member. A toner accommodating unit is configured to have a toner supplying opening opposing a surface of the developer bearing member at a position adjacent to and upstream of the developer accommodating unit in a developer conveying direction. The toner in the toner accommodating unit is taken into the developer on the developer bearing member based on a toner density of the developer on the developer bearing member by a movement of the developer with a conveyance of the developer on the developer bearing member. According to the preferred embodiment, the developing device further includes a peeling off member that opposes, without contacting, a surface of the developer bearing member in the developer accommodating unit so as to peel off an upper layer part of the developer carried and conveyed by the developer bearing member.
In the developing device of the above preferred embodiment, an upper layer part of the developer on the developer bearing member, which is conveyed to a position opposing the developer accommodating unit, is peeled off by the peeling off member in the developer accommodating unit. The developer thus peeled off by the peeling off member is mixed with the developer accommodated in the developer accommodating part. By this mixing of the developer, the toner taken in the developer is stirred and distributed, and at the same time the charging of the toner is accelerated, and thereby the distribution and the charging amount of the toner in the developer on the developer bearing member are made uniform. A part of the developer thus mixed in the developer accommodating unit is carried by the developing sleeve at the downstream side of the peeling off member in the developer conveying direction together with the developer carried by the developer bearing member without having been peeled off by the peeling off member. The developer on the developer bearing member is regulated by the developer regulating member so as to have a predetermined layer thickness and is then used for development. Because only the upper layer part of the developer on the developer bearing member is peeled off, the effect of peeling off the developer relative to the conveyance of the developer on the developer bearing member is suppressed and thereby the self toner density control is stably performed without being deteriorated.
According to another preferred embodiment of the present invention, the developing device includes a developer stirring member which opposes, without contacting, a surface of the developer bearing member in the developer accommodating unit at a position adjacent to the toner accommodating unit so as to stir a part of the developer carried and conveyed by the developer bearing member, where the toner has been taken into the developer from the toner accommodating unit.
In the developing device of the above another preferred embodiment of the present invention, the upper layer part of the developer on the developer bearing member entered into the developer accommodating part is peeled off by the developer stirring member which opposes, without contacting, a surface of the developer bearing member in the developer accommodating unit at the position adjacent to the toner accommodating part. By this stirring of the developer, the mixing of the developer on the developer bearing member and that accommodated in the developer accommodating unit is performed. Further, by this stirring of the developer, the toner which has been taken in the upper layer part of the developer on the developer bearing member is stirred and mixed with the developer such that the toner density of the developer is made uniform. Thus, the developer in which the toner density is made uniform is carried by the developer bearing member and conveyed to a developing area so as to be used for development.
According to still another preferred embodiment of the present invention, the developing device includes a flexible plate-like member which opposes, without contacting, a surface of the developer bearing member in the developer accommodating unit at a position adjacent to the toner accommodating unit so as to contact a part of the developer carried and conveyed by the developer bearing member, where the toner has been taken into the developer from the toner accommodating unit.
In the developing device according to the above another still preferred embodiment, a flexible plate-like member opposing a surface of the developer bearing member at a position adjacent to the toner accommodating part in the developer accommodating unit contacts the supplied toner existing on the upper layer part of the developer on the developer bearing member entered into the developer accommodating part. When the supplied toner contacts the flexible plate-like member, the flexible plate-like member is bent or restored by a pressure force of the toner attempting to pass the flexible plate-like member, thereby preventing the excessive toner existing on the upper layer part of the developer on the developer bearing member from being conveyed to an area at the downstream side of the flexible plate-like member in the developer conveying direction together with the developer on the developer bearing member. Further, because of the flexible plate-like member, the supplied toner existing on the upper layer part of the developer on the developer bearing member is uniformly distributed in a width direction of the developer bearing member, which is perpendicular to the developer conveying direction.